In a conventional vehicle air conditioner, air to be blown into a vehicle compartment (passenger compartment) is heated by using exhaust heat generated in the vehicle, such as engine-cooling water, as a heating source. In this case, when the temperature of engine-cooling water is low, a sufficient heating capacity cannot be obtained.
To overcome this problem, in a vehicle air conditioner described in JP-A-5-96931, high-pressure refrigerant discharged from a compressor is supplied to an interior heat exchanger so as to heat air to be blown toward a passenger compartment. However, the refrigerant discharged from the interior heat exchanger is decompressed by a fixed throttle such as a capillary tube. Accordingly, when a thermal load at a high-pressure side or a low pressure side changes, the pressure of high-pressure refrigerant is changed; and therefore, the heating capacity of the interior heat exchanger is greatly changed.